Owing to its almost unlimited resources and harmlessness, carbon is an excellent material in view of resources and environmental problems. Carbon materials have diverse forms of interatomic bonding, and a variety of crystal structures are known, such as diamond, diamond-like carbon, graphite, fullerene and carbon nanotubes. Above all, diamond-like carbon having an amorphous structure (amorphous carbon) is expected to be applied in each industrial field, because of its high mechanical strength and good chemical stability.
However, general amorphous carbon films have electric resistance in a range of semiconductors to insulators. In order to further widen the use of amorphous carbon, it has been requested to impart electric conductivity to amorphous carbon. One of the use applications of the amorphous carbon is a bipolar plate for a fuel cell. An example of a single cell of a polymer electrolyte fuel cell is schematically shown in FIG. 5. The left diagram of FIG. 5 shows arrangement of its respective constituent elements before laminated and the right diagram of FIG. 5 shows a laminated state of these elements. A single cell 1 is constituted by an electrolyte membrane 1a, and a pair of electrodes (an air electrode 1b and a fuel electrode 1c) sandwiching the electrolyte membrane 1a from both sides. Bipolar plates 2 have channel-formed surfaces 2b, 2c on which a plurality of channels are formed. The bipolar plates 2 are housed in resin bipolar plate frames 3 and laminated so that the air electrode 1b and the channel-formed surface 2b face each other and the fuel electrode 1c and the channel-formed surface 2c face each other, respectively. Thus, gas flow passages sectioned by electrode surfaces and the channels are formed between the electrodes and the bipolar plates, and fuel gas and oxygen gas, which are reaction gases of the fuel cell, are efficiently supplied to the electrode surfaces.
In the fuel cell, the fuel gas and the oxygen gas need to be separately supplied to the entire electrode surfaces without mixed with each other. Therefore, the bipolar plates need to be gas tight. Furthermore, the bipolar plates need to collect electrons generated by reaction and have good electric conductivity in order to serve as electric connectors for connecting adjoining single cells when a plurality of single cells are stacked. Moreover, because electrolyte membrane surfaces are strongly acidic, the bipolar plates are demanded to have corrosion resistance.
Therefore, as a bipolar plate material, it is common to use graphite plates. However, since the graphite plates break easily, the graphite plates have a problem with workability in producing bipolar plates by forming a plurality of gas passages, making surfaces flat and so on. On the other hand, because metallic materials have good workability as well as good electric conductivity and especially titanium and stainless steel have good corrosion resistance, the metallic materials can be used as bipolar plate materials. However, since metallic materials having good corrosion resistance are easily passivated, the metallic materials have a problem of increasing internal resistance of a fuel cell and causing a voltage drop. Therefore, a bipolar plate formed by coating a surface of a metal substrate with an electrically conductive amorphous carbon film is attracting attention.
Examples of a process for imparting electric conductivity to amorphous carbon include a process for adding a metal to amorphous carbon (See Japanese Unexamined Patent Publication Nos. 2002-038268 and 2004-284915). However, the added metal may become a cause of corrosion or, when used in contact with another metal, may become a cause of adhesion, so inherent chemical stability of amorphous carbon can be damaged.
Therefore, Japanese Unexamined Patent Publication No. 2008-004540 discloses an amorphous carbon film having good electric conductivity and corrosion resistance, obtained based on a finding that electric conductivity can be imparted to an amorphous carbon film by increasing ratio of a structure comprising carbon having an sp2 hybrid orbital (Csp2) by controlling the content of hydrogen and carbon having an sp2 hybrid orbital (Csp2) in amorphous carbon. Specifically, the film is an amorphous carbon film containing carbon as a main component wherein the film contains more than 0 at. % and not more than 30 at. % of hydrogen and if necessary, not more than 20 at. % of nitrogen, and when the total amount of the carbon is taken as 100 at. %, the amount of Csp2 is not less than 70 at. % and less than 100 at. %. In this amorphous carbon film, delocalization of π electrons is promoted due to a large ratio of Csp2 in the entire carbon and molecular termination by C—H bonds (n bonds) is suppressed due to a decrease in hydrogen content. As a result, this amorphous carbon film exhibits a high electric conductivity. Besides, this amorphous carbon film is amorphous and does not have such electric anisotropy as exhibited by single crystal graphite.
Moreover, one of the functions requested to a bipolar plate of a fuel cell is a function to rapidly discharge water generated by a battery reaction at an electrode from gas passages in order to ensure passages of fuel gas and oxygen gas. Water remaining in the gas passages causes a decrease in power generation performance of the fuel cell. Therefore, an improvement in wettability of surfaces of the gas passages is important to performance of the fuel cell. For example, Japanese Unexamined Patent Publication No. 2011-108547 mentions that an amorphous carbon film is designed to contain 20 at. % of nitrogen in order to improve hydrophilicity of the amorphous carbon film.